# Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs

## TL;DR

This study investigates the relationship between twin kHz QPO emission radii and neutron star co-rotation radii in 12 NS-LMXBs, revealing that QPOs form inside the co-rotation radius and are influenced by magnetic fields and accretion rates.

## Contribution

It provides new insights into the formation regions of twin kHz QPOs and their relation to neutron star spin and magnetic field effects in low mass X-ray binaries.

## Key findings

- Twin kHz QPO emission radii are inside the co-rotation radius.
- Most twin kHz QPOs cluster around 15-20 km radius.
- QPO upper frequency exceeds NS spin frequency by over 10%.

## Abstract

We analyze the relation between the emission radii of twin kilohertz quasi-periodic oscillations (kHz QPOs) and the co-rotation radii of the 12 neutron star low mass X-ray binaries (NS-LMXBs) which are simultaneously detected with the twin kHz QPOs and NS spins. We find that the average co-rotation radius of these sources is r_co about 32 km, and all the emission positions of twin kHz QPOs lie inside the corotation radii, indicating that the twin kHz QPOs are formed in the spin-up process. It is noticed that the upper frequency of twin kHz QPOs is higher than NS spin frequency by > 10%, which may account for a critical velocity difference between the Keplerian motion of accretion matter and NS spin that is corresponding to the production of twin kHz QPOs. In addition, we also find that about 83% of twin kHz QPOs cluster around the radius range of 15-20 km, which may be affected by the hard surface or the local strong magnetic field of NS. As a special case, SAX J1808.4-3658 shows the larger emission radii of twin kHz QPOs of r about 21-24 km, which may be due to its low accretion rate or small measured NS mass (< 1.4 solar mass).